1. Field of the Invention
The present invention relates in general to a projection system and, more particularly, to a structural improvement in such a projection system for reducing the convergence angles for red, green and blue color beams using a dichroic mirror, thus to increase the horizontal and vertical visible angles, reduce color shift of a color picture displayed on a screen, improve uniformity of the color picture, reduce convergence compensating amount, as well as circuit compensating amount or keystone compensating amount and to improve luminance due to reduction of the keystone compensating amount.
2. Description of the Background Art
With reference to FIGS. 1 to 4, there is shown a typical projection system. As shown in these drawings, the typical projection system 10 has a support part 10a and a screen part 10b in its bottom section and in its top section respectively.
In the interior of the support part 10a, three beam projection tubes, that is, a red beam projection tube 11 having a first projection lens 11' in its front section, a green beam projection tube 12 having a second projection lens 12' in its front section and a blue beam projection tube 13 having a third projection lens 13' in its front section, are arranged in a horizontal line in the center of the interior of the support part 10a, thus to form an in-line arrangement of the beam projection tubes. In the above projection system, the red beam projection tube 11, the green beam projection tube 12 and the blue beam projection tube 13 have a first side optical axis 11", a center optical axis 12" and a second side optical axis 13", respectively.
Placed in the screen part 10b above the three beam tubes 11, 12 and 13 is a reflection mirror 14 that reflects the beams of the three optical axes 11", 12" and 13". A screen 15 is placed in front of the reflection mirror 14, so that the screen 15 displays thereon the three beams reflected by the reflection mirror 14.
The first and second side optical axes 11" and 13" are inclined at an angle .alpha. of inclination relative to the center optical axis 12" as shown in FIG. 3. The inclination angle .alpha. is so-called a convergence angle of the projection system 10. In order to compensate for the deviation of the three beams caused by the convergence angle .alpha., each of first and second projection tubes 11 and 13 of the projection system 10 is provided with Scheimflug angle .alpha.' as shown in FIG. 4.
As the red beam tube 11, the green beam tube 12 and the blue beam tube 13 are arranged in the in-line type as described above, the first and second side optical axes 11" and 13" of the red and blue beam projection tubes 11 and 13 should be deflected toward the center optical axis 12" of the green beam projection tube 12 at a predetermined angle. However, the deflection of the first and second side optical axes 11" and 13" increases compensating amount, thus causing a difficulty in compensation. Even when the color picture is fortunately accurately focused on the screen 15 as a result of compensation, there may be another problem of color shift in the color picture displayed on the screen 15, A further problem of the typical projection system is resided in that the system not only narrows the vertical and horizontal visible angles but also causes nonuniform color of the picture displayed on the screen 15.
In addition, when using a short focus lens in order to reduce the length of the beam path, the total conjugation length (TCL) is shortened, thus increasing the compensating amount due to the intervals between the three beam projection tubes 11, 12 and 13, increasing color shift of the picture displayed on the screen 15, and increasing convergence compensating amount as well as keystone compensating amount. Both increases in the convergence compensating amount and in the keystone compensating amount cause a circuit to be overloaded. Particularly, the increase in the convergence compensating amount causes deterioration of both luminance and resolution of the picture displayed on the screen.